1. Field of the Invention
The present invention relates to circuit breaker systems, and specifically to multipole circuit breaker systems for multiphase circuits which utilize sequential contact operation for improved safety.
2. Discussion of the Related Art
Many formats for electrical current delivery are currently in use. For example, a common end-user standard in the United States uses two electrical conductors. This system (commonly known as a single-phase system) may utilize a single neutral conductor in conjunction with a single electrified or "hot" conductor. The neutral conductor remains essentially at an earth ground voltage, and the hot conductor commonly provides a sinusoidal time varying voltage (e.g. 120 Volts at a frequency of 60 Hz).
Another format of electrical current delivery is commonly known as a three-phase system. This format includes three electrified or "hot" conductors, and a fourth (neutral) conductor. Each hot conductor generally provides an identical sinusoidal waveform, but the voltage supplied by each conductor is phase-shifted .+-.120.degree. from the two remaining hot conductors. This standard is very widely used, especially in industrial applications.
In three-phase systems in the United States, this neutral conductor is caused to remain substantially at an earth-ground voltage potential. (In some instances a low voltage may occur between the neutral conductor and the local earth-ground potential in the neighborhood of the load, but this does not normally pose a safety threat.)
In contrast, some three-phase power supply systems in some other countries (notably France) provide a fourth neutral conductor which is not caused to remain substantially at an earth-ground voltage potential. This fourth conductor may be referred to as a "floating neutral." The voltage between the neutral conductor and an earth-ground potential may pose a serious safety threat should a person come into contact with conductors at the potential of the neutral conductor.
Therefore, in countries using such systems, it is a common practice to cause the floating neutral to be disconnected when the power is removed from the electrical load. (In the U.S., where the neutral pole is substantially at an earth ground voltage potential, this is not allowed by the national electrical code requirements.)
An important element in many electrical protection and safety systems is a circuit breaker device. A circuit breaker device is designed to open the electrical circuit in the event of an unacceptably high current flow. It is important to realize that a circuit breaker device can only offer protection from dangerous current levels, not dangerous voltage levels. Although many dangerous situations may involve both high voltage and high current levels, this need not be the case. In the case of an electrical equipment fault, the circuit breaker device offers no protection unless a low resistance electrical path is present to allow the excessive current to return from the electrical fault. For example, a circuit breaker device will offer no protection if an electrical fault occurs to an ungrounded chassis (which provides no electrical return path). Therefore, it is generally desirable in an electrical environment to cause conductive structures, such as chassis or enclosures, to remain at or near an earth ground voltage potential.
In the three-phase systems in which the neutral conductor is "floating" (not substantially at an earth ground voltage potential), it is desirable to place a circuit breaker device in the neutral conducting path. This has two functions. One function is to protect against unacceptably high current levels on the floating neutral conductor. Unacceptably high current may occur when the floating neutral pole experiences a fault condition to an earth ground potential. Another function is to prevent exposure to the possible high voltages on the neutral conducting path, after a circuit breaker has tripped on one of the other poles.
Multipole circuit breakers typically comprise several interconnected single-pole units positioned adjacent each other. (An individual circuit breaker must be provided for each electrified or "hot" conductor, as an overcurrent condition may occur on any of the individual phases.) The manual switching handles of the respective breakers may be connected to each other for simultaneous manual actuation of all poles. Alternatively, or in addition to connecting the respective manual switching handles, a mechanism may be provided to trip open all of the breaker poles simultaneously, whenever any one of them is tripped.
Mechanical tolerances in the various linkages used to join the multiple poles may mean that the several breakers make or break contact in a sequential fashion. This inadvertent sequential contact operation is undesirable, especially in a floating neutral system.
Since the floating neutral system requires a circuit breaker device in the neutral leg, a problem may be created if a hot pole is closed prior to the neutral pole, or if the neutral pole is opened prior to any nonneutral pole. The voltage applied by one or more hot poles prior to the connection of the neutral pole can be dangerous, since, as previously mentioned, the neutral pole will oftentimes provide an electrical return path in the event of an electrical fault. Therefore, personnel may be briefly exposed to dangerous voltages because the circuit breaker device cannot operate until the return path (i.e., the neutral pole) is closed.
Even if a multipole circuit breaker system can achieve substantially simultaneous contact operation of its several poles, it can be seen that in the interest of safety, it would be beneficial to have a floating neutral multipole circuit breaker system which assures that the neutral pole breaker will make contact prior to and break contact subsequent to the remaining single pole breakers. A system such as this will thereby eliminate any of the previously mentioned problems.
An example of a circuit protective device can be seen in U.S. Pat. No. 3,949,336, to Dietz. The device disclosed in the Dietz patent protects against a ground fault condition using sequential contact operation. The neutral contact is made first and broken last to prevent the hot contacts from being connected without the neutral contact.
The present invention is directed, in part, to overcoming the above-mentioned problems associated with known circuit breaker devices.